Abstract

The G protein-coupled receptor kinases (GRKs) are best known for their role
in phosphorylating and desensitising G protein-coupled receptors (GPCRs). The
GRKs can also regulate signalling downstream of other families of receptors and are
now known to have a number of non-receptor substrates and binding partners. Here I
identify RhoAGTP, Raf1 and ERK2 as novel binding partners of GRK2 and report a
previously unsuspected function for this kinase. GRK2 acts as a RhoA-activated
scaffold protein for the ERK MAP kinase cascade downstream of the epidermal
growth factor (EGF) receptor. The ability of GRK2 to bind to Raf1, MEK1 and
ERK2 is dependent on RhoAGTP binding to the catalytic domain of the kinase,
however, while RhoAGTP binding is common to all of the ubiquitously expressed
GRKs, the ability to act as a RhoA-regulated Raf/MEK/ERK scaffold is specific to
GRK2. GRK2 over-expression in HEK-293 cells potentiates EGF-induced ERK
activation in a Rho-dependent fashion. Conversely, depleting GRK2 expression by
RNAi reveals that GRK2 is required for EGF-induced thymidine incorporation in
vascular smooth muscle cells (VSMCs). Rho-dependent ERK MAP kinase
scaffolding by GRK2 may therefore have an important role in the vasculature, where
increased levels of GRK2 and RhoA have been associated with hypertension.